The present invention relates in general to substrate manufacturing technologies and in particular to methods and apparatus for in situ wafer temperature monitoring by electromagnetic radiation emission.
In the processing of a substrate, e.g., a semiconductor wafer or a glass panel such as one used in flat panel display manufacturing, plasma is often employed. As part of the processing of a substrate (chemical vapor deposition, plasma enhanced chemical vapor deposition, physical vapor deposition, etc.) for example, the substrate is divided into a plurality of dies, or rectangular areas, each of which will become an integrated circuit. The substrate is then processed in a series of steps in which materials are selectively removed (etching) and deposited (deposition) in order to form electrical components thereon.
In an exemplary plasma process, a substrate is coated with a thin film of hardened emulsion (i.e., such as a photoresist mask) prior to etching. Areas of the hardened emulsion are then selectively removed, causing parts of the underlying layer to become exposed. The substrate is then placed in a plasma processing chamber on a substrate support structure comprising a mono-polar or bi-polar electrode, called a chuck. Appropriate etchant source gases (e.g., C4F8, C4F6, CHF3, CH2F3, CF4, CH3F, C2F4, N2, O2, Ar, Xe, He, H2, NH3, SF6, BF3, Cl2, etc.) are then flowed into the chamber and struck to form a plasma to etch exposed areas of the substrate.
Among the set of process variables that can be adjusted to optimize the plasma process are gas composition, gas phase, gas flow, gas pressure, RF power density, voltage, magnetic field strength, and wafer temperature. Although in theory it may be beneficial to optimize each variable to each processing step, in practice it is often difficult to achieve. Substrate temperature, for example, is important since it may subsequently affects plasma selectivity by changing the deposition rate of polymeric films, such as poly-fluoro-carbon, on the wafer surface. Careful monitoring may minimize variation, allow a wider process window for other parameters, and improve process control. However, in practice it may be difficult to directly determine temperature without affecting the plasma process.
One technique, for example, measures the substrate temperature by a temperature probe. Referring now to FIG. 1A, a simplified cross-sectional view of a plasma processing system is shown, in which a temperature probe is used to determine wafer temperature. Generally, an appropriate set of etchant source gases is flowed into chamber 100 and struck to form a plasma 102, in order to etch exposed areas of substrate 104, such as a semiconductor wafer or a glass pane. Substrate 104 generally sits on chuck 106. Electromagnetic radiation 110a, 110b, and 110c (collective 110) produced by plasma 102, in combination kinetic energy transferred by the plasma itself, causes substrate 104 to absorb thermal energy. In order to determine substrate temperature, probe 108 extends from beneath substrate 104 to contact the substrate. However, probe 108 may also dislodge the wafer from the chuck, and subsequently ruin a costly wafer.
Another technique is the measurement of Infrared (IR) radiation from the substrate with a conventional pyrometer. Generally, heated materials emit electromagnetic radiation in the IR region. This region generally comprises a wavelength range from 8 to 14 μm, or a frequency range from 400 to 4000 cm−1, where cm−1 is known as wavenumber (1/wavelength) and is equivalent to frequency. Measured IR radiance can then be used to calculate substrate temperature by using Planck's radiation law for blackbody radiation.
Referring now to FIG. 1B, a simplified cross-sectional view of a plasma processing system is shown, in which a conventional pyrometer is used to determine wafer temperature. As in FIG. 1A, an appropriate set of etchant source gases is flowed into chamber 100 and struck to form a plasma 102, to etch exposed areas of substrate 104. Substrate 104 generally sits on a chuck 106. Plasma 102 may also produce a spectrum of electromagnetic radiation 110, some of which is generally IR. It is this radiation (along with kinetic energy transferred by the plasma itself) that may cause substrate 104 to absorb thermal energy. Substrate 104, in turn, also generates IR radiation corresponding to its temperature. However, since substrate's 104 IR radiance is generally substantially smaller than that of the plasma, a pyrometer 120 may not be able to distinguish between the two. Hence, the calculated temperature would be approximately that of the background plasma itself and not of the substrate.
Still another technique is the use of an interferometer to measure a change in substrate thickness due to absorbed thermal energy. Generally, an interferometer measures a physical displacement by sensing a phase difference of an electromagnetic beam reflected between two surfaces. In a plasma processing system, an electromagnetic beam may be transmitted at a frequency for which the substrate is translucent, and positioned at an angle beneath the substrate. A first portion of the beam may then reflect on the substrate's bottom surface, while the remaining portion of the beam may reflect on the substrate's top surface. Referring now to FIG. 1C, a simplified cross-sectional view of a plasma processing system is shown, in which an interferometer is used to determine wafer temperature. As in FIG. 1A, an appropriate set of etchant source gases is flowed into chamber 100 and struck to form a plasma 102, to etch exposed areas of substrate 104, such as a semiconductor wafer or a glass pane. Substrate 104 generally sits on chuck 106. Plasma 102 produces electromagnetic radiation 110, some of which is IR. This radiation (along with kinetic energy transferred by the plasma itself), causes substrate 104 to absorb thermal energy and expand by an amount 118. An electromagnetic beam transmitter 108, such as a laser, transmits beam 112 at a frequency for which substrate 104 is translucent. A portion of the beam reflection then reflects 114 at point 124 on the substrate's bottom surface, while the remaining portion of beam 116 reflects at point 122 on the substrate's top surface. Since the same beam 112 is reflected at two points 122 and 124, the resulting beams 114 and 116 may be out of phase, but otherwise identical. Interferometer 130 can then measure the phase shift and determine the substrate thickness 118. By taking successive measures, a change in substrate thickness may be determined. However, a change to substrate thickness may only be used determine a corresponding change in temperature, and not a specific temperature. Furthermore, since the transmitter is also located in the plasma processing system, it can become damaged by plasma 102, and may also produce contaminants that may affect manufacturing yield.
Because of these difficulties, substrate temperature is normally inferred from the rate of heat dissipation from the plasma processing system. Generally, some type of cooling system is coupled to the chuck in order to achieve thermal equilibrium once the plasma is ignited. That is, although substrate temperature in generally stabilized within a range, its exact value is commonly unknown. For example, in creating a set of plasma processing steps for the manufacture of a particular substrate, a corresponding set of process variables, or recipe, is established. Since the substrate temperature may not be directly measured, optimizing a recipe is difficult. The cooling system itself is usually comprised of a chiller that pumps a coolant through cavities in within the chuck, and helium gas pumped between the chuck and the wafer. In addition to removing the generated heat, the helium gas also allows the cooling system to rapidly calibrate heat dissipation. That is, increasing helium pressure subsequently also increases the heat transfer rate.
Referring now to FIG. 1D, a simplified diagram of temperature 180 versus time 181 is shown for a substrate 188, after the plasma is ignited. Initially, the substrate is at ambient temperature range 182. As the plasma is ignited, the substrate absorbs thermal energy during a stabilization period 184. After a period of time, the substrate temperature stabilizes at range 186. Since the duration of stabilization period 184 may be a substantial portion of the total plasma processing step, decreasing stabilization period 184 may directly improve yield. If the substrate temperature could be directly measured in a plasma processing system, the cooling system could be optimized to minimize stabilization period 184.
In addition, depending on the plasma processing activity, its duration, or its order relative to other steps, a different amount of heat may be generated and subsequently dissipated. Since as previously explained, substrate temperature may directly affect the plasma process, first measuring and then adjusting the substrate temperature would allow plasma processing steps to be better optimized.
Furthermore, the physical structure of the plasma processing chamber, itself, may change. For example, pollutants may be cleaned from the plasma processing system by striking the plasma without the substrate. However, the chuck is no longer shielded by the substrate, and is subsequently etched. As the cleaning process is repeated, the substrate's surface roughness increases, modifying its heat transfer efficiency. Eventually, the cooling system cannot adequately compensate, and the recipe's parameters are invalidated. Since it is often impractical to determine when this point is exactly reached, the chuck is generally replaced after a certain amount of operational hours, which in practice is normally only a fraction of its useful life. This can both increase productions costs, since an expensive chuck may be needless replaced, and reduces yield, since the plasma processing system must be taken offline for several hours to replace the chuck.
In addition, recipe parameters may need to be adjusted since an otherwise identical piece of fabrication equipment may be installed at a different time, or is used to a different degree, its maintenance cycle does not necessarily match that of the others. The recipe parameters may need to be adjusted when moving the process to a newer version of the plasma processing system, or when transferring the process to a plasma processing system that can process a larger substrate size (e.g., 200 mm to 300 mm). Ideally, it would be beneficial to maintain the same recipe parameters (e.g., chemistry, power, and temperature). However, since wafer temperature is inferred and not measured, the process may need to be substantially adjusted through trial and error in order to achieve a similar production profile. In view of the foregoing, there are desired improved methods and apparatus for in situ wafer temperature monitoring.